The murine B cell tumor, BCL1, which expresses both IgM and IgD on its surface, can be induced by mitogens and T cell factors to undergo a series of differentiation events in vitro. These include (1) maturation into atmore mature B cell; (2) differentiation into a IgM secreting cell; and (3) "switching" into an IgG secreting cell. Using appropriate nucleic acid probes, DNA and RNA from these populations will be analyzed in order to correlate the external induction events to molecular events such as chromatin DNAse sensitivity, DNA methylation and translocation, and RNA processing of secreted and membrane forms. DNA from isotype switched BCL1 cells will be cloned and characterized to determine the precise mechanism and points of recombination.